1. Field of the Invention
The present invention relates to intrusion detection devices such as, for example, of the passive infrared type, and more particularly to wide angle optical systems for such.
2. Description of the Prior Art
Conventional intrusion detection devices commonly have passive infrared sensors which recognize changes in received infrared energy to indicate movement of a person into and within a field of view. However, these sensors can only cover an arc of about 60 degrees because passive infrared sensors are characterized by a lambertain distribution of sensitivity. In particular, manufacturers typically specify that the sensitivity decreases as the cosine of the angle from the optical axis. Thus, at an angle of 60 degrees from the optical axis, the sensitivity would be only 50 percent what it is at the center. Even if it were not for the lambertain distribution of sensitivity, the housing of commercially available sensors restrict the field of view. For Example, one such commercially available sensor has an enclosure restricting the field of view to 46 degrees from the optical axis, and another to 55 degrees.
One way to improve the effective field of view of an infrared sensor is to employ an optical system to direct the electromagnetic radiation emanating from any one of a plurality of discrete fields of view onto a single sensor, or closely spaced multiples of such sensors. For example, FIG. 1 shows a typical device having a Fresnel lens system with an array of lens segments tilted with respect to each other so as to have their own discrete fields of view, and so as to focus parallel radiation in that field of view on a common focal point so as to provide an infrared sensor located at the common focal point with a plurality of discrete, sector-shaped fields of view. As an infrared source moves into and out of these fields of view, a sudden change in the level of infrared radiation is detected by the sensor and a signal is generated to a controlled circuit. Even with such a lens system, and a sensor capable of a 90 degree field of view, the greatest angle which can be monitored is approximately 90 degrees because the angle of incidence of radiation on the sensor is approximately the same as the angle of incidence into the device, and because the standard sensor unit only supplies small signals when receiving radiation from a large angle of incidence. However, it is often desirable for many uses to be able to monitor a significantly larger angular range than 90 degrees.
Several attempts have been made to increase the effective field of view of intrusion detectors using reflective surfaces. For example, see U.S. Pat. No. 4,268,752 which issued on May 19, 1981 to Herwig et al, U.S. Pat. No. 4,644,147 which issued on Feb. 17, 1987 to Zublin, and U.S. Pat. No. 4,703,171 which issued on Oct. 27, 1987 to Dahl et al. Each patent describes an optical system for expanding the field of view of an intrusion detection device up to approximately 180 degrees. While such coverage is generally sufficient to monitor a single rectangular-shaped space, it is often desirable to be able to monitor larger angles so as, for example, to be able to cover an L-shaped area from the inside corner.
Additionally, each of these prior patents presents a unique problem overcome by the intrusion detection device according to the present invention. Herwig U.S. Pat. No. 4,268,752 has a significant obscurration of the field of view by a mirror, which is not used to extend or deflect the field of view. The system of Zublin U.S. Pat. No. 4,644,147 rotates the image in space before it impinges the sensors, and provides reflectors which are not readily accessable for the purpose of changing the fields of view. Kahl U.S. Pat. No. 4,703,171 has a 15 degree "dead" space along the optical axis.